Use the menu item Frames | Frame properties or double-click the frame thumbnail in the frame list below the editor.
The duration should be a multiple of 10 milliseconds.
In an animated GIF we can specify a disposal method for each frame, which determines what will happen when the frame's time slice is over. The available disposal methods are:
You can set the disposal method in the Frames | Frame properties dialog.
With Frames | Convert GIF frames to keyframes you can convert a GIF to an equivalent animation where each frame has the Restore background disposal method. This is useful if you want to extract the "effective" frames from a GIF.
Use the test window: choose Icon | Test (Ctrl+Shift+T) or press on the toolbar above the image editor panel, just like when testing an icon.